Photochromic glasses characteristically darken under the influence of short wavelength radiation, and fade when the radiation source is removed. Such glasses have become well-known since their disclosure in U.S. Pat. No. 3,208,860 (Armistead et al.).
Initially, the active photochromic ingredient was crystals of a silver halide, other than the fluoride. Subsequently, glasses containing copper and cadmium halides were found to be photochromic, but the silver halide glasses have remained the commercial choice.
Continuing efforts were made to improve the early glasses with respect to both their photochromic properties and their other properties necessary for ophthalmic use. These efforts led to U.S. Pat. No. 4,190,451 (Hares et al.). This patent discloses an R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 base glass containing, as essential constituents for photochromism, Ag 0.15-0.3%, 0.1-0.25% Cl, 0.1-0.2% Br and 0.004-0.02% CuO by weight. The patent also discloses the possibility of adding up to one percent transition metal oxides, such as CoO, NiO and Cr.sub.2 O.sub.3, and up to five percent rare earth metal oxides, such as Er.sub.2 O.sub.3, as glass colorants.
A composition for a commercial, photochromic sunglass was developed on the basis of the Hares et al. patent teachings. This glass has a base glass composition, as calculated in parts by weight from the glass batch, of
______________________________________ SiO.sub.2 56.6 Na.sub.2 O 4.1 Al.sub.2 O.sub.3 6.3 K.sub.2 O 5.7 B.sub.2 O.sub.3 18.1 ZrO.sub.2 4.6 Li.sub.2 O 1.8 TiO.sub.2 1.9 ______________________________________
The glass contains photochromic elements as follows:
______________________________________ Ag 0.24 Cl 0.21 Br 0.17 CuO 0.007 ______________________________________
The glass also has 0.190% NiO and 0.024% CoO added to impart a fixed tint.
A 2 mm. thick lens of this glass has a faded transmittance of 69%. When fired in hydrogen to provide parameters designed to produce a pleasing coloration, the faded transmittance of the glass is reduced to 45+/-2%. However, a lens having an even lower, faded transmittance after hydrogen treatment was found to be desirable.
This could be achieved by a more extended hydrogen treatment in terms of time and/or temperature. However, such extended treatment resulted in a color shift that created a very undesirable cosmetic appearance. Also, after UV darkening, as by sunlight, a sunglass lens failed to meet the ISO/ANSI sunglass requirements.
To achieve the desired transmittance with acceptable color, it became necessary to apply a combination of vacuum-deposited coatings, including a neutral density, durable, thin film coating, to the H.sub.2 -fired glass. This expedient effectively provided a faded transmittance of about 26% and a UV-darkened transmittance of about 12% at 25.degree. C. However, the extra processing added considerable cost to the lens.
The present invention is predicated on discovery that the need for a coating can be avoided by modifying and controlling the amounts of the colorant oxide. The resulting lens, in a standard, 2.0+/-0.1 mm. thickness, satisfies the ISO/ANSI requirements, as well as other ophthalmic requirements. These include a refractive index of about 1.53, an ability to be chemically strengthened to meet the FDA ball-drop test and a cosmetically acceptable appearance.
It is then a basic purpose of the present invention to produce an improved, H.sub.2 -fired, darker sunglass lens blank.
Another purpose is to provide a fixed tint, photochromic glass from which such lens blank can be produced, and from which, in the unfired state, a brown photochromic glass is produced.
A further purpose is to provide a sunglass lens that possesses the properties of a lens having a vacuum-deposited coating, while avoiding the costly, coating procedure.
Another purpose is to provide a fixed tint, photochromic glass lens that has an acceptable color and diminished transmission in the undarkened state, but still satisfies ISO/ANSI sunglass requirements when darkened.
A still further purpose is to accomplish the forgoing purposes while modifying only the colorant contents in a commercial glass.